This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Resistive switching properties of a self-compliance resistive random access memory
device in cross-point architecture with a simple stack structure of Ir/TaOx/W have been investigated. A transmission electron microscope and atomic force microscope
were used to observe the film properties and morphology of the stack. The device has
shown excellent switching cycle uniformity with a small operation of ±2.5 V and a
resistance ratio of >100. The device requires neither any frorming-process nor current
compliance limit for repeatable operation in contrast to conventional resistive random
access memory devices. The effect of bottom electrode morphology and surface roughness
is also studied. The improvement is due to the enhanced electric field at the nanotips
in the bottom electrode and the defective TaOx switching layer which enable controlled filament formation/rupture. The device area
dependence of the low resistance state indicates multifilament formation. The device
has shown a robust alternating current endurance of >105 cycles and a data retention of >104 s.

Keywords:

RRAM; Cross-point; TaOx; Self-compliance

Background

Resistive random access memory (RRAM) is the most promising candidate for the next-generation
nonvolatile memory technology due to its simple structure, excellent scalability potential
(<10 nm), long endurance, high speed of operation, and complementary metal-oxide-semiconductor
(CMOS) process compatibility
[1-7]. RRAM in cross-point architecture, in which top and bottom electrodes are placed
at right angle to each other, is very attractive as it offers high-density integration
with 4 F2, F being the minimum feature size area; three-dimensional (3D) stacking; and cost-effective
fabrication
[8,9]. Switching uniformity is one of the important properties which require practical
realization of cross-point devices with large array size. So it is necessary to investigate
the factors affecting switching uniformity. Various binary transition metal oxides
such as HfOx[5,6,10-12], TiOx[13,14], TaOx[2,7,15-18], AlOx[19-21], ZrOx[22-24], WOx[25], etc. as a switching material are reported for RRAM application. Among them, recently,
TaOx has attracted much attention
[26] owing to its superior material and switching properties such as having two stable
phases
[15], high thermal stability
[18], small difference between the free energies of low and high resistance states
[26], CMOS compatibility, long endurance
[2], and high switching speed
[7]. So far,a cross-point resistive switching memory device in an Ir/TaOx/W structure has not yet been reported.

In this study, self-compliance-limited and low-voltage-operated resistive switching
behaviors with improved switching cycle uniformity in a simple resistive memory stack
of Ir/TaOx/W in cross-point architecture are reported. The physical properties of switching
stack and bottom electrode morphology have been observed by transmission electron
microscope (TEM) and atomic force microscope (AFM) analyses. The improvement is due
to the defective switching layer formation as well as the electric field enhancement
at the nanotips observed in the bottom electrode surface which results in controlled
and uniform filament formation/rupture. The self-compliance property shows the built-in
capability of the device to minimize the current overshoot during switching in one
resistance (1R) configuration. The device has shown an alternating current (ac) endurance
of >105 cycles and a data retention of >104 s.

Methods

A cross-point resistive memory stack in an Ir/TaOx/W structure have been fabricated on SiO2 (200 nm)/Si substrate. The fabrication steps are schematically depicted in Figure
1. A sputter-deposited W layer of approximately 250 nm was patterned using photolithography
and wet etching methods in order to get W bottom electrode (BE) bars. A deposition
power and pressure of 100 W and 5 mTorr, respectively, were used for the W layer deposition,
and sizes (width) of W bars were between 4 and 50 μm. After an additional lithography
patterning step for lift-off using a second mask at right angle to define top electrode
(TE) bars, a TaOx switching layer was deposited by an electron beam evaporator system using pure Ta2O5 granulates under a high vacuum of 2 × 10−6 Torr. To avoid any atmospheric oxidation/contamination effects on the TaOx switching layer, an Ir layer of about 50 nm as TE was immediately deposited on the
TaOx layer using an Ir target by a sputtering system. The rf power and working pressure
were 50 W and 5 mTorr, respectively, and the sizes of the TE bars were the same as
those of the BE bars (4 to 50 μm). Finally, the lift-off process was performed to
get the cross-point devices. The sizes of the cross-points were in the range of 4 × 4
to 50 × 50 μm2. An optical microscope image of such a cross-point with an area of 4 × 4 μm2 is shown in Figure
2. The TE and BE bars at right angles along with the contact pads are shown. The electrical
characterizations have been performed using an Agilent 4156 C precision semiconductor
parameter analyzer (Santa Clara, CA, USA) in voltage sweep mode at room temperature
and ambient conditions. The voltage applied on TE and BE was electrically grounded
during measurement.

Results and discussion

In order to confirm the fabricated RRAM device stack and film thickness, cross-sectional
TEM images were acquired, as shown in Figure
3. The size of the cross-point is approximately 6 × 6 μm2 (Figure
3a). The TaOx switching layer sandwiched between W (BE) and Ir (TE) metal electrodes is clearly
visible, as shown in Figure
3b. The amorphous TaOx/WOx layer thickness on the top of W BE is approximately 20 nm. The WOx layer is formed during the fabrication process. The columnar growth of both metal
electrodes is also evident in the TEM image. Further, the thickness of the stack layers
is higher on the top of W BE than on the sidewall due to the sputtering deposition.
The thickness of the TaOx/WOx layer on the sidewall is approximately 10 nm, which is thinner than that of the top
side (approximately 20 nm). This suggests that the conducting filament will be formed
on the sidewall rather than the top side.

The current–voltage (I-V) characteristics of the cross-point device in the Ir/TaOx/W structure are shown in Figure
4a. The initial resistance of the pristine device was higher than that of the high
resistance state (HRS), and the first set voltage was almost similar to the subsequent
set voltage (curve not shown here). Such type of forming step-free resistance memory
devices is particularly attractive for practical realization because of its cost-effectiveness
and reduction in circuit complexity. The BE morphology and smaller thickness of TaOx on the sidewalls resulted this forming step-free behavior. The bipolar I-V curves of all the subsequent 100 consecutive direct current (dc) sweep cycles with
highlighted 1st and 100th cycles are shown in Figure
4a. As no obvious difference between the first and the last cycle is observed, the
device shows excellent switching cycle uniformity with tight distribution in low resistance
state (LRS) and HRS. The small dispersion is required for large cross-point arrays.
Further, unlike conventional RRAMs, this device does not require any current compliance
limit for operation which indicates its built-in current overshoot reduction capability
which is helpful in obtaining long pulse endurance without the use of a transistor
as current limiter. The self-compliance behavior is due to the high bulk resistance
of the device which resulted owing to the WOx and electrically formed interface layer near the TE during the first cycle of device
break-in
[27]. Also, the I-V curve of the LRS is nonlinear and the resistance of the LRS is high (>100 kΩ). In
order to investigate the current conduction mechanism in both LRS and HRS, the switching
I-V curve in the positive-bias region is replotted in a log-log scale, as shown in Figure
4b. Two linear regions in LRS as well as in HRS were identified with the different
slopes of 1.6 and 2.3, and 3.9 and 6.6, respectively. The slope values suggest that
the conduction mechanism in both LRS and HRS is trap-controlled space-charge-limited
conduction (TC-SCLC). At smaller voltage, traps are unfilled and hence current is
small, whereas at higher voltage, the current increases fast (I∝V2.3 in LRS and I∝V6.6 in HRS) due to trap filling. Oxygen vacancies might serve as trap sites. Further,
as the I-V curve of LRS is nonlinear, the oxygen vacancy conducting filament might not be dense;
generally, ohmic conduction is observed in a thick and dense filament
[28]. The switching mechanism can be attributed to the formation/rupture of the oxygen
vacancy conducting filament upon the application of appropriate electric field.

The improvement in the switching can be co-related with the surface morphology of
the W bottom electrode observed in the AFM image, as shown in Figure
5. The enhancement of the electric field at the tips can help in controlled filament
formation/rupture which leads to the uniformity in the switching parameters. Similar
results are reported by Huang et al.
[29]. These two types of BEs with different surface roughness were prepared by controlling
the deposition method (sputtering or PECVD) and parameters such as power or working
pressure during sputtering. The AFM images of smooth and nanotip BE surfaces are shown
in Figure
5. Figure
5a,c shows two-dimensional (2D) or planeviews of surface roughness for the smooth and
nanotip samples, respectively. Figure
5b,d shows 3D views of the smooth and nanotip samples, respectively. The average (Ra) and root mean square (rms; Rq) surface roughness values of smooth and nanotip BE surfaces are found to be 1.05
and 1.35 nm, and 3.35 and 4.21 nm, respectively. These self-assembled nanotips are
observed from our W BE surface. Experimental data shows that the switching cycle uniformity
and pulse endurance were greatly improved in the devices with nanotip BE surface.
This is due to the controlled and easy formation/rupture of the conducting filament
during switching owing to the enhanced electric field at the nanotips observed in
the AFM image. Also, it is expected that the film will be more defective on the nanotip
BE surface. Due to these reasons, the cross-point memory device shows almost forming-free
or low-voltage operation. Figure
6 shows the device-to-device cumulative probability plot of LRS and HRS of cross-point
memory devices with different sizes of 4 × 4, 20 × 20, and 50 × 50 μm2, respectively. More than 20 cross-points of each size have been measured randomly
across the 4-in. wafer. Most of the devices show resistive switching with an HRS/LRS
ratio of >10. The average resistance of LRS increases by decreasing the device size
from 50 × 50 to 4 × 4 μm2. This might be due to the multifilament formation which is more probable when the
device size is large, which is due to the nonuniform deposition of the switching layer
on the sidewalls. It is expected that device-to-device uniformity can further be improved
under a better facility. In order to confirm the nonvolatility of LRS and HRS, the
resistance of both states is monitored with time and plotted in Figure
7a. The read voltage was +0.2 V. As can be seen, both LRS and HRS are fairly stable
for more than 104 s at room temperature. Figure
7b shows the ac endurance capability of our cross-point memory device. The device was
successively programmed and erased at +2.5/−2.5 V with 500-μs pulse, respectively,
and read after each program/erase event at +0.2 V, as schematically shown inside Figure
7b. The data of every such program/erase event is recorded and plotted. The read pulse
width was 10 ms. Due to every cycle read, variation of HRS/LRS with cycle-to-cycle
is observed, which is slight read disturb. Further study is necessary to overcome
this problem. However, an excellent ac endurance of more than 105 cycles is achieved. A high resistance value of LRS (approximately 1 MΩ) might be
useful in fabricating large size arrays by suppressing the leakage current from unselected
cells and reduce the active power consumption.

Figure 7.Data retention and endurance. (a) Good data retention and (b) excellent ac endurance with every cycle reading of >105 are obtained. All switching devices have such a long endurance.

Conclusions

Improvement in the resistive switching and self-compliance behaviors of a forming-free
resistive memory stack of Ir/TaOx/W in a cross-point structure has been obtained. The cross-sectional TEM image confirms
the amorphous TaOx/WOx film. The AFM image shows the presence of nanotips on the W bottom electrode surface.
The device has shown excellent switching uniformity during 100 consecutive dc sweeps
with set/reset voltages of ±2.5 V and a resistance ratio of >100. The self-compliance
behavior which comes from the bulk resistance of the stack shows the built-in capability
of the device to minimize current overshoot during switching. The improvement in the
switching is attributed to the formation of a defective switching layer and bottom
electrode surface morphology with nanoscale tips which can enhance the electric field
resulting in the uniform formation/rupture of the oxygen vacancy conducting filament.
The device has exhibited an ac cycle endurance of >105 cycles and a data retention of >104 s. It is expected that this self-compliance, low-voltage-operated cross-point resistive
memory device could be useful for the development of future nanoscale nonvolatile
memory devices.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

AP carried out the fabrication, measurement, and analysis of the cross-point memory
devices, and he wrote the manuscript under the instruction of SM. DJ and SS measured
the memory devices under the instruction of SM. All authors contributed to the revision
of the manuscript, and they approved it for publication.

Acknowledgements

This work was supported by the National Science Council (NSC), Taiwan, under contract
number NSC-102-2221-E-182-057-MY2.